Radiation curable adhesives which cure by free radical mechanisms have gained wide acceptance in a number of product assembly applications. Cationic curing systems enjoy several advantages over free radical systems. First, cure is not instantaneous and thus the possibility exists to irradiate the sample and then make the bond. This feature obviates the need for one substrate being radiation transparent, which many are not. Secondly, since there is no termination mechanism other than neutralization of the active acid catalyst by impurities, the cure proceeds into the “dark” (i.e. non-irradiated) regions of the sample. Thus, if impurities are minimized the entire adhesive sample will eventually cure as long as one part of it is irradiated. Consequently thick films are readily cured at low dose (fast production speeds).
Most cationic systems use cycloaliphatic epoxide monomers or vinyl ethers. Kraton Polymers™ recently developed a system based on epoxidized block copolymers, with the epoxy groups residing almost exclusively in the polyisoprene blocks. These systems enabled highly flexible and even pressure sensitive radiation cured adhesives based on cationic cure chemistry. However, these systems have not gained commercial acceptance in part because cure has not been reproducible.
A need remains for a cationic curable PSA that can produce tapes of consistent quality in a production environment. The current invention addresses this need.